Standard methods for measuring a physical value use measuring of a characteristic parameter of a sensing device, which characteristic parameter is dependent on the physical value to be measured. For example, for measuring a temperature or pressure, a resistance of a temperature or pressure dependent sensing element may be measured by measuring current through or voltage over a sensing element. The accuracy of the measurement is proportional to the accuracy of a reference voltage or reference current and to the accuracy of the sensing element.
Most known standard methods use analog-to-digital conversion based on a dual slope integration method, which makes the conversion speed slower.
For example U.S. Pat. Nos. 3,875,503 and 4,217,543 both describe a principle using a dual slope integration cycle. This means that two integration frames are needed for getting the desired result.
In U.S. Pat. No. 3,875,503 an integrator first integrates an unknown analog signal for a predetermined time period and then integrates a reference signal of inverse polarity with respect to the analog signal until the integrator is restored to its initial value. Digital counting means digitally measure a time interval ending when the integrating means-is restored to its initial value.
In U.S. Pat. No. 4,217,543, during a first cycle, a voltage drop over a reference resistor is integrated, and during a second cycle, a voltage drop over a sensing resistor is integrated.
In U.S. Pat. No. 5,655,305, the sensing element is a part of a bridge resistor network. The analog-to-digital converter is again using a dual slope integration method.
In EP-1102405 use is made of a three-port sigma-delta modulator having an input port, a reference port and an offset port. Only one conversion cycle is used for evaluating an unknown resistance value. A bridge topology is used for the sensor, from which a differential signal to be fed to the input port is derived. A differential signal driving the reference port is derived as a fraction of the bridge supply voltage to make accuracy of the measurement independent on it. An input at the offset port is used to indirectly correct the sensor (bridge) offset: the offset voltage must be determined before the measurement cycle.
If a parasitic resistance is present in any of the above measurement devices, such as a parasitic resistance due to a sensing element boding contact for example, then this cannot be remedied.